Dynamical Controls of Mesoscale Water Vapor Variability in the Tropical Western Pacific

Idealized simulations of radiative-convective equilibrium (RCE) with cloud resolving models have been used as a numerical laboratory to understand how diabatic processes can drive convective clustering, which in turn leads to significant drying of the free troposphere and increase in spatial humidity variability. These processes, such as feedbacks between radiation, clouds and water vapor have been found to have relevance for numerous large-scale modes of convective organization, such as the width of the upward branch of the Hadley cell, ENSO and the Madden Julian Oscillation. However, the controls of water vapor associated with convective variability on the sub-1000km mesoscale are less well known. We adopt a simple multivariate analysis technique previously used to assess convective organization in RCE, and apply it to analyze convective organization and its impact on column integrated humidity (precipitable water, PW) variability for mesoscale-size boxes in the tropical western Pacific warm pool region. We find that in periods where the SST gradients are very limited, convection remains mostly random and the horizontal humidity gradients are small on these scales. Instead in the periods of the year where the zones are subject to a weak meridional gradient of SST ($> 10^{-3} K km$^{-1}$), convection is mostly aggregated with much larger humidity gradients associated with an increase of clear sky OLR exceeding 10 W m$^{-2}$. However, this situation is regularly disturbed by intermittent, multi-day episodes of homogeneous "random" convection and small humidity gradient. By using an index based on the SST-PW relationship, we construct a composite of 44 such events over a 4 year period which shows that they are associated with a westward-propagating, convectively-coupled Rossby wave-like mode that is symmetric about the equator. An independent multivariate (SST-PW) rotated EOF analysis confirms this, indicating the robustness of the result. We hypothesize that the longer-term variations in an convective organization index which was directly related to the tropics-wide energy budget (Bony et al.) may be driven by the frequency of occurrence of these westward propagating modes, that seem to act as a primary control on mesoscale water vapor variability in the warm pool region in the boreal winter and spring months.